For heavy duty hoisting or driving, a wound rotor induction motor is often used with the motor usually arranged to start at maximum torque. For general purpose applications, a variety of types of squirrel cage motors are used. Particularly in the former case, but also in the latter, it is important to be able to protect a motor from a stall condition, especially at motor start up when heavy current can be drawn by a motor.
As far as applicant is aware, the existing start-stall and stall protection designs allow the full starting current to be provided to the machine on startup, irrespective of whether the machine is moving or stationery. This results in considerable wastage of power, as well as the stressing of the motor to its full overload limit before a stall trip protection operates.
Generally, if it is desired to see if an induction motor is in fact moving, a tache generator on the motor shaft, or other shaft sensor, is used to indicate rotation.